Merge TypeParamRefs in orDominator

Author: noti0na1

compiler/src/dotty/tools/dotc/core/TypeOps.scala

@@ -271,6 +271,13 @@ object TypeOps:
               tp1.derivedSelect(pre1 | pre2)
             case _ => fallback
           }
+        case tp1: TypeParamRef =>
+          tp2 match {
+            case tp2: TypeParamRef
+            if (tp1.binder eq tp2.binder) && (tp1.paramNum == tp2.paramNum) =>
+              tp1
+            case _ => fallback
+          }
         case AndType(tp11, tp12) =>
           mergeRefinedOrApplied(tp11, tp2) & mergeRefinedOrApplied(tp12, tp2)
         case _ => fail

tests/pos/i19789.scala

@@ -0,0 +1,5 @@
+type Kinded[F[_]] = F[Any] | F[Nothing]
+
+def values[F[_]]: Vector[Kinded[F]] = ???
+
+def mapValues[F[_], T](f: Kinded[F] => T): Vector[T] = values[F].map { case x => f(x) }